with all the recent discussion about moving the question about shuttleports and the city cap limet on planets has come up a few times.
its very simple.
The 3 main worlds of Corellia, Naboo, and Tatooine are able to hold 20 and the moons and adventure worlds of Talus, Rori, Dantooine, and Lok can hold up to 50 cities.
It is important to note that there is also a cap on the number of cities that can achieve a certain rank. While the core worlds of Corellia, Naboo, and Tatooine can have a total of 20 cities, only 15 can be at rank 3 or above and only 10 can be at rank 4 or above.
The moons/adventure planets of Talus, Rori, Dantooine, and Lok can have 30 cities at or above rank 3 and 20 at or above rank 4.